Viruses mutate all the time, including the novel coronavirus that’s caused the global pandemic. But a variant that emerged in southeast England in September is causing particular concern, leading to an emergency lockdown in London over Christmas and prompting countries including Canada, France and Germany to halt flights and suspend rail links to the U.K..

1. Why is this mutation alarming?

Preliminary analysis in the U.K. suggests it may be as much as 70% more transmissible than other circulating SARS-CoV-2 strains and contributing to a spike in cases in the country. Dubbed the “B.1.1.7 lineage,” the strain has acquired 17 mutations compared to its most recent ancestor — a faster rate of change than scientists typically observe. Some of those are in key areas involved in the virus’s ability to infect cells. The World Health Organization is working to understand the extent to which the virus may spread more easily, along with human behavioral factors that may be driving transmission. Maria van Kerkhove, the organization’s technical lead on Covid-19, told the BBC on Dec. 20 that the WHO is also investigating whether the mutation causes more severe illness and can evade the antibodies generated by vaccination.

2. What’s known about when it emerged?

The two earliest known specimens were collected in late September in London and the nearby county of Kent, and cases continued to be found through early December. One reason the new variant proliferated was that its emergence coincided with the festive season traditionally associated with increased family and social mixing. Scientists have hypothesized that the strain may have resulted from the transmission of the virus from a chronically infected patient.

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